1. Field of the Invention
This invention relates to solid electrolytic capacitors using solid electrolytes which exhibit good stability, reliability and prolonged life under high temperature and high humidity conditions. The invention also relates to methods for manufacturing such solid electrolytic capacitors.
2. Description of the Prior Art
In recent trends toward digitalization of circuits of electric and electronic appliances, there is a strong demand for capacitors which exhibit a low impedance in a high frequency range and have a small size and a high capacitance.
Known capacitors used in a high frequency range include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. These capacitors are so large in size that a difficulty is involved in attaining a large capacitance.
On the other hand, a certain type of capacitor is known as having a large capacitance. Such capacitors include, for example, an aluminium dry electrolytic capacitor and an aluminium or tantalum solid electrolytic capacitor. However, the electrolytes used in these capacitors, such as liquid electrolytes and manganese dioxide solid electrolytes are high in resistance, so that a satisfactorily lower impedance in a high frequency range cannot be obtained.
In recent years, there have been proposed and used organic semiconductors having high conductivity and anodizability instead of the manganese dioxide solid electrode. Typical organic semiconductors include 7,7,8,8-tetracyanoquinodimethane complex salts, which may be hereinafter referred to simply as TCNQ salt or salts. As set forth in Japanese Patent Publication No. 56-10777 and Japanese Kokai No. 58-17609, aluminium solid electrolytic capacitors using the TCNQ salts are remarkably improved in frequency and temperature characteristics, resulting in a low leakage current characteristic. Although the TCNQ salts are organic semiconductive materials, they exhibit high thermal stability. Accordingly, it is considered that the capacitors using the TCNQ salts have a longer life at high temperatures than known dry electrolytic capacitors.
Moreover, solid electrolytic capacitors using another type of organic conductive polymer have been proposed, for example, in Japanese Kokai Nos. 60-37114 and 60-244017. In such capacitors, heterocylic monomers, such as pyrrole, thiophene and the like, are electrolytically polymerized using support electrolytes, thereby forming, on an anode, a highly conductive polymer layer containing the anions of the support electrolyte as a dopant.
In addition, Japanese Kokai 1-310529 describes a solid electrolytic capacitor wherein a conductive polymer layer is formed by depositing a metal or a conductive metal compound or oxide on a dielectric film and subjecting a polymerizable monomer to electrolytic polymerization while contacting an electrode for electrolytic polymerization with the metal or the metal compound.
However, solid electrolytic capacitors have generally the problems that the ability of repairing defects of a dielectric film with the solid electrolyte is low and that the adhesion between the electrolyte and the dielectric film is low. These problems become more pronounced especially when conductive polymers are used as the solid electrolyte. In general, solid electronic elements are used by encacement in resin casings in order to prevent the elements from suffering mechanical damages or from deterioration by the attack of oxygen or moisture. With solid electrolytic capacitors using conductive polymer electrolytes, however, the element undergoes a shrinking stress which occurs during a curing or cooling step of the resin casing, leading to the problem that the electrolyte layer is apt to separate from the dielectric film, or the dielectric film is liable to be damaged. This results in a lowering of the capacitance or an increase of the leakage current. Since the organic solid electrolyte in low is the film-repairing ability, the increase of the leakage current characteristic cannot be stopped even after the element has been subsequently aged. To avoid this, attempts have been often made to reduce the stress by the use of resins having rubber elasticity but are not in success because there arise such problems as mentioned above when such a capacitor is placed under abruptly varying temperature conditions. Thus, it is not possible to prevent degradation of capacitor characteristics.
In particular, when used as a solid electrolyte, TCNQ salts have the problem that a specific resistance unfavorably increases upon application of the salt and that the adhesion to an anode metal foil is poor.
On the other hand, with capacitors using a high conductive polymer layer formed by electrolytic polymerization, it is technically difficult to form the electrolytically polymerized layer on a dielectric layer. Although it is known that an electrolytically polymerized layer of high conductivity is formed on a conventional anode such as, for example, platinum, carbon or the like, by using a solution of a heterocylic compound, such as pyrrole, thiophene or derivatives thereof, and an appropriate support electrolyte, the dielectric film is insulative in nature and any electric current does not pass through the dielectric film. In this sense, it will be principally difficult to electrolytically form the conductive polymer layer on the dielectric film.
It is also known that an electrolytically polymerized layer is readily formed on a conductive layer formed on the dielectric field through an electrode for electrolytic polymerization. However, the resulting capacitor is disadvantageous in that the dielectric film is liable to degrade by the interaction between the conductive layer and the dielectric film and that the life of the capacitor under high temperature and high humidity conditions is poor.